1. Field of the Invention
This invention relates to methods for manufacturing a dilithium salt of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) (hereinafter referred to as Li2DMcT) and Li2DMcT dihydrate. The Li2DMcT dihydrate manufactured by this invention is an industrially very useful substance which is used widely, for example, as a photosensitive material for photos, a dye, an inhibitor, a surface-treating agent for metals, a raw material for additives to lubricating oils, a monomer for high refractive index plastics, an electrode material for lithium batteries, and a starting material or intermediate for medicines and agricultural chemicals.
2. Description of the Related Art
DMcT dilithium salt (abbreviated to Li2DMcT hereafter) is very hygroscopic and deliquescent, and its isolation is very difficult; therefore nothing has been reported on its properties, and its handling has been hard. In addition, neither has the existence of its dihydrate been clearly reported nor it has been even suggested. Besides it is known that thiol compounds are easy to oxidize and that addition of alkali to them further facilitates their oxidation. This is true also of Li2DMcT; Li2DMcT is easily oxidized by air. This has raised a problem in trying isolation of Li2DMcT; the crystallization doesn""t go well when the compound""s purity is low, and yet the application of a purification process to the compound makes its purity lower because of the oxidation.
The need for size and weight reduction of portable equipment and the consideration on the environment and energy issues have promoted very much the research and development of high energy density secondary batteries for years.
Particularly secondary lithium batteries have been developed remarkably. While at present the positive electrode material is mainly a metal oxide such as lithium cobaltate, etc., research on the use of organic compounds as a positive electrode material has been attracting attention on the grounds that such an electrode may achieve higher energy density than that of today""s batteries using metal oxides. Of the organic compounds thiols are the most promising, and attempts to make use of the reversibility of thiols"" oxidation and reduction reaction for the second battery have been made. One of those attempts was made by Koyama et al., who reported on the use of DMcT as a positive electrode material (Nature, vol.373, No.6515, p.598-600,1995); the battery in this report achieved high energy density by using as positive electrode a composite electrode made of conductive polyaniline and the thiol compound. In another example, i.e., in tokukaihei (Japanese Patent Application Publication No. 4-26704), Ueno et al. reported the use of lithium thiolate as a positive electrode material, and metallic aluminum or its alloy together with a carbon material as a negative electrode. Besides the current-voltage characteristic is lowered when an aluminum alloy containing 30 atom % lithium is used as a negative electrode; from this result, there will be a problem of battery performance degradation if metallic lithium is used as a negative electrode.
The inventors of the present invention developed various polythiol compounds and studied batteries using those compounds as a positive electrode material, but could not obtain a favorable battery because of the following problems:
1) bad cycle characteristics
2) odor of the compound used
3) generation of hydrogen gas during the initial charging and discharging
4) low heat stability of the compound used
5) in the case of a liquid polythiol compound, vaporization of the polythiol in the positive electrode-drying process
The use of the lithium salts of those polythiols as a positive electrode-active material and metallic lithium as a negative electrode-active material resulted in further lowering the cycle characteristics and also posed a problem of hydrogen gas generation during the initial charging and discharging.
The objective of this invention is to solve the problems listed above and provide methods for manufacturing Li2DMcT and Li2DMcT dihydrate, the compounds of which having as mentioned above a wide range of uses as a photosensitive material for photos, a dye, an inhibitor, a surface-treating agent for metals, a raw material for additives to lubricating oils, a monomer for high refractive index plastics, an electrode material for lithium batteries, and a starting material or intermediate for medicines and agricultural chemicals.